The surface treatment of workpieces using plasma and gas lasers is an industrial process in which a plasma is produced, in particular in a plasma chamber, by direct current or by a high-frequency alternating signal having a working frequency in the range from some 10 kHz into the GHz range. The plasma chamber is connected via further electronic components, such as inductors, capacitors, lines or transformers, to a high-frequency generator (HF generator). Those further components may form oscillating circuits, filters or impedance matching circuits. The electrical load impedance of the plasma chamber (the plasma) which arises during the process depends on the conditions in the plasma chamber and may vary greatly. In particular, the properties of the workpiece, electrodes, and gas ratios are a consideration. High-frequency generators have a limited working range with regard to the impedance of the connected electrical load. If the load impedance leaves a permissible range, the high-frequency generator may be damaged or even destroyed.
For that reason, an impedance matching circuit (matchbox) is generally required which transforms the impedance of the load to a nominal impedance of the generator output (typically 50Ω). If there is a mismatch, it is not possible for the full generator power to be supplied to the load. Instead, some of the power is reflected. In the region of the nominal impedance there is an impedance range, that is, a range of transformed load impedances, in which the generator operates in a stable manner and is not damaged. If the transformed load impedance is outside that nominal impedance range, damage to the generator and instability of the generator may occur as a result of reflected power.
Some impedance matching circuits have a fixed setting or a predefined transforming effect, that is, they consist of electrical components, especially inductors and capacitors, that are not altered during operation. That is appropriate particularly when operation always remains constant, for example in the case of a gas laser. In other impedance matching circuits, at least some of the components of the impedance matching circuits are mechanically variable. For example, in motor-driven rotary capacitors, the capacitance can be varied by altering the arrangement of the capacitor plates relative to one another.
Broadly speaking, three impedance ranges may be associated with a plasma. Before ignition, very high impedances are present. In normal operation, i.e. when working with plasma in accordance with its intended use, lower impedances are present. In the case of undesired local discharges (arcs) or in the case of plasma fluctuations, very low impedances may occur. In addition to the three impedance ranges identified, further, special conditions with other associated impedance values may occur. If the load impedance changes abruptly and if in that case the load impedance or the transformed load impedance leaves a permissible impedance range, the generator and/or the transmission circuitry between the generator and the plasma chamber may be damaged.
PIN diodes are electronic components constructed similarly to a pn diode. In contrast to the pn diode, however, the p-doped layer is not in direct contact with the n-doped layer, but a weakly doped or undoped i-layer lies in between. That i-layer is intrinsic. Since it contains only few charge carriers, however, it has a high resistance. In the forward direction, the PIN diode operates similarly to a normal semiconductor diode. In the case of PIN diodes, however, the lifetime of the charge carriers in the undoped i-layer (i-region) is particularly high. When charge carriers are brought into the i-layer by a forward current, the PIN diode remains constantly conductive even when a high frequency is superposed on the forward current and, as a result, short voltage pulses are periodically applied in the reverse direction. In that state, a PIN diode behaves like a resistor. In the completely switched-on state, voltage drops in the order of magnitude of the forward voltage of the semiconductor material used still occur.
If the diode is operated by applying a direct voltage in the reverse direction, a space charge region of differing width is produced in the p-region and the i-region. Owing to the wide space charge region in the i-region, those diodes are suitable for high reverse bias voltages. For a superposed high frequency, a reverse-biased PIN diode essentially represents a capacitor formed by the depletion layer.
Owing to its behavior as a resistor at high frequencies, a PIN diode may be used as a dc-controlled ac voltage resistor or as a high-frequency switch. In that case, a high-frequency alternating current and a direct current in the forward direction or a dc voltage in the reverse direction may be superposed, thereby enabling the resistance of the i-region to be controlled.
In some impedance matching circuits the mechanically variable reactances (e.g., rotary capacitors, roller inductors) are replaced by capacitor or inductor arrangements controlled by PIN diodes. For example, U.S. Pat. No. 7,226,524 discloses switching in capacitors via PIN diodes in normal matching mode, U.S. Pat. No. 4,486,722 discloses short-circuiting coil sections or switching in capacitors in normal matching mode, and U.S. Pat. No. 5,654,679 describes varying a capacitor by selecting capacitor subunits. However, a great number of PIN diodes with associated activation elements may be required, resulting in an expensive circuit. In addition, high losses may occur since switched-on PIN diodes are not without resistance and reverse-biased PIN diodes are not unrestrictedly good insulators. Still further, some arrangements may not be fast enough to prevent damage to the HF generator or the PIN diodes if there is a sudden change in impedance. Furthermore, parts of inductors carrying HF current that are short-circuited by PIN diodes or inductors short-circuited by PIN diodes and magnetically coupled to inductors carrying HF current may produce losses due to induced currents.